Dr. Drive on Cam Ring Failure
Cam ring failures are common in radial piston hydraulic motors and are not specific to a particular model of a final drive -- it can be found in any of the radial piston motors used on Bobcat, CAT, CASE, and New Holland machines. In this Shop Talk Blog post featuring Dr. Drive, you will learn what a cam ring failure is, how it happens, and what can be done about it. We'll start with a quick review on cam rings ...
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What Is a Cam Ring?
In a radial piston hydraulic motor or final drive, the cam ring is key to achieving motion and torque. As hydraulic fluid enters the radial piston hydraulic motor, it applies pressure to the cam piston/roller assembly. As the pistons are pressurized, they force the metal rollers against the cam ring, causing rotation. This involves metal-to-metal contact and is one of the instances where the lubrication properties of hydraulic fluid is very important.
The surface finish of the portion of the cam ring that comes in contact with the rollers must be extremely smooth and extremely accurate. Significant damage to the surface will impair the efficiency of the final drive or hydraulic motor. Damage usually occurs as a result of abrasive contaminants becoming trapped between the piston rollers and the cam ring during operation.
Damaged Cam Ring on Hydraulic Motors
One of the hydraulic motors we recently received had a cam ring that was torn up badly. The inside surface of a cam ring should be smooth, but this was scratched, gouged, dinged, abraded -- in short, scarred up very badly. Along with the damaged cam ring, we also discovered severely damaged pistons and piston rollers. In short, the piston rollers had destroyed the cam ring. If something like this happens to you, you will end up with something wrong with your final drive that can be expensive to fix (if it can be fixed at all).
Symptoms
Needless to say, this severely impacted the performance of the final drive in question, gradually getting worse until the final drive simply quit running. Symptoms would likely include noise, poor performance, and eventual failure.
Repair or Replace
Sadly, the cam ring by itself cannot really be fixed. It might be possible to grind and polish the interior surface or possibly re-machining the surface, but that depends on the severity of the damage. If it is damaged badly enough, the only recourse is to replace it.
Next we have the issue of the damaged pistons and rollers. These were chewed up pretty badly, and they really cannot be repaired. They will need to be replaced.
If the parts cannot be repaired, they will be replaced. Our first approach is to take a look at the same model motor cores that we have in stock and check for one that has an undamaged cam ring. We would look for a motor that experienced a failure in no way related to the cam assembly, such as a final drive that experienced an axle failure. The pistons and rollers would be new parts.
Causes
In order to prevent this type of failure, it is important to know what causes it to happen. Typically we see this kind of damage coming from hydraulic system contamination. Any time there is a failure, it is very important that the hydraulic system is flushed and all filters (include the case drain filter) be replaced.
Contaminants such as metal shavings or flakes are often generated during a major failure, and if they are not flushed out of the system then they will immediately get to work on destroying any new components -- including final drives -- that you install.
Note that there are other possible causes, such as normal wear and tear resulting in metal fatigue or a loss of hardness in the rollers and cam rings, but most of the cam ring failures we see can be traced back to contaminants.
Detecting and Preventing Cam Ring Failure
Any time your radial piston motor, whether it's on a CASE skid steer loader or some other brand, begins to make some new noises and you notice a significant loss of power, remember that it could be issues with the cam ring. The more quickly the issue is detected and addressed, of course, the less damage will result. To prevent this type of failure, make sure to change your hydraulic fluid and filters according to manufacturer recommendations. If your system experiences a failure, always flush the hydraulic fluid and change out the filters.
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